A conventional hermetic compressor (hereinafter referred to simply as “compressor”), which is formed of a compressing mechanism and an electric motor both accommodated in a housing hermetically welded, is disclosed in Japanese Patent Unexamined Publication No. H06-159274. This compressor is free from refrigerant leakage or water invasion, so that it has been widely used in air-conditioners or refrigerators because of its high reliability.
FIG. 5 shows a sectional view of a conventional compressor. In FIG. 5, compressing mechanism 53 and electric motor 54 are accommodated in cylindrical housing 52 to form a compressor of a high-pressure dome model. Housing 52 is equipped with discharging tube 56 at its upper end for discharging compressed refrigerant gas.
Compressing mechanism 53 is a rolling piston model and rigidly mounted to housing 52, and connected with sucking tube 55 for feeding the refrigerant gas into housing 52. Compressing mechanism 53 is coupled to motor 54 with driving shaft 57, so that it is driven by motor 54.
Motor 54 is disposed above compressing mechanism 53 and connected to hermetic terminal 58 welded at the upper end of housing 52. Terminal 58 is used for powering, and an external source powers motor 54 through this hermetic terminal 58, which is excellent in pressure resistance and airtight performance.
Driving shaft 57 is equipped with a centrifugal pump (not shown) and a lubrication path (not shown), and disposed extending through compressing mechanism 53. The centrifugal pump is disposed at a lower end of driving shaft 57, so that it can pump up refrigerating machine oil pooled at the bottom of housing 52. The lubrication path is formed inside shaft 7 along the axial direction, and supplies the oil pumped up by the centrifugal pump to the respective sliding sections.
The foregoing compressor supplies the refrigerating machine oil pooled in housing 52 to compressing mechanism 53 and its bearings for lubrication. The refrigerating machine oil pooled in housing 52 is discharged together with compressed refrigerant gas from the compressor. Under normal conditions, the oil circulates through a refrigerant circuit and returns to the compressor, so that an amount of the oil is maintained in housing 52. However, the amount of the oil varies depending on the operation, and it sometimes becomes short and fails in lubrication.
To the contrary, if the oil is pooled excessively, a large amount of the refrigerating machine oil is discharged together with compressed refrigerant gas from compressor 51, thereby inviting lower performance of a heat exchanger as well as of the refrigerator.
Several ideas have been proposed to the problem discussed above, e.g. oil surface 59 in housing 52 is sensed by a sensor for detecting a shortage or an excess of the oil pooled, so that the compressor is protected. One of those ideas is disclosed in Japanese Patent Unexamined Publication No. 2001-12351: Detection of a lower oil surface 59 starts a protecting action such as halting the operation of compressor 51 or collecting the refrigerating machine oil from the refrigerant circuit, thereby preventing the compressor from being damaged.
Detecting a position of oil surface 59 in housing 52 needs sensors disposed in housing 52 around the oil surface and signals to be transmitted from the sensors to the outside of housing 52. To achieve the detection, a conventional compressor has employed two thermistors in a detector, and a difference in temperatures of the two thermistors has told the oil surface position. The conventional compressor is also equipped with a hermetic terminal at the upper section of the housing, and the thermistors are connected to the hermetic terminal for transmitting the signals to the outside of the housing. The foregoing structure needs two thermistors and connections between the termistors and the hermetic terminal, so that the structure becomes complex and causes poor operation, and invites lower reliability because of a possible disconnection. Some of conventional compressors employ a single thermistor, which however simply measures a temperature, so that an accurate detection of the oil surface cannot be expected.
The foregoing publication (No. 2001-12351) also discloses that an oil surface sensor, which is integrally formed of a detector for detecting an oil surface in the housing and hermetic terminals, is mounted on a side-wall of the housing. However, since the side-wall is shaped like a cylinder, the mounting of the sensor onto the side-wall will invite a defect in airtight performance due to distortion, or causes a failure in airtight performance due to a collision in assembling the compressor.
Further, the oil surface sensor is mounted in the housing at a place corresponding to the lower limit of the oil surface, and after a detection of the lower limit of the oil surface, the oil surface cannot rebound immediately although an oil-surface rebounding action is taken. This delay further lowers the oil surface. This phenomenon sometimes causes serious damage to the compressor.
On top of that, employment of sensors for detecting simply a temperature of the refrigerant gas and that of the refrigerating machine oil sometimes shows temperatures similar to each other depending on an operating condition, and an operation during a transition period particularly causes the sensors to malfunction.